This invention relates generally to gaseous fuel cells and is particularly directed to an arrangement for electrically coupling and separating and sealing adjacent cells in a stocked array of molten carbonate fuel cells.
Gaseous fuel cells are comprised of an anode electrode spaced apart from a cathode electrode with an electrolyte disposed in the inter-electrode space. Each electrode includes a catalyst layer on the electrolyte side thereof. On the nonelectrolyte side of the anode electrode is circulated a fuel gas, while on the nonelectrolyte side of the cathode electrode is circulated an oxidant gas. The electrodes are constructed to permit a respective reactant gas to diffuse therethrough and to come in contact with the electrolyte and the catalyst layer thereby causing an electrochemical reaction wherein ions travel from one electrode to the other through the electrolyte and electrons travel from one electrode to the other via an external circuit.
In a fuel cell power plant a plurality of individual fuel cells are electrically coupled in series through conductive, gas impervious plates separating adjacent cells in forming a stacked array of fuel cells. These separator plates, in combination with the electrodes adjacent thereto, generally define the reactant gas passages which transit each fuel cell. The individual cells thus function as separate batteries coupled in series to provide an electrical output.
This type of energy source is referred to as a molten carbonate fuel cell because the electrolyte is in the form of a liquid at typical cell operating temperatures in the range of 550.degree. C. to 750.degree. C. The electrolyte is generally mixed so as to form a matrix with an inert particulate material which remains solid during cell operation to maintain spacing between the electrodes. These cells are thus characterized by high operating temperatures and highly corrosive internal elements.
The high temperatures and corrosive environment of molten carbonate fuel cells confront the fuel cell designer with difficult structural and fabrication problems. For example, changes in cell component thickness result from manufacturing tolerances or from characteristics exhibited by the cell components in response to operation of the cell. Thus, for example, at the high operating temperatures of the cell, the electrodes and electrode supports undergo thermal expansion, increasing the compressive forces thereon. Cell components undergo creep over time which tends to lessen the compressive forces and may cause cell components such as the separator plates to pull away from the cell and impair the sealing of the active cell areas. In addition, the thermal environment within the cell may substantially reduce the electrical and thermal conductivity between cell components and between adjacent cells resulting in low efficiency cell operation.
The first application referenced above discloses a fuel cell separator plate with compressive sealing rails each comprising a marginal flange of the separator plate folded back upon itself and cooperating with the plate to form a channel in which is inserted a stack of thin metal sheets. The thin metal sheets, each of which has a slight deviation from true flatness so as to afford a degree of compressibility, are bonded to one another, to the sealing flange, and to the plate in a laminated structure having the desired thickness. While this arrangement is capable of accommodating variations in the thickness of the associated fuel cell components due to creep, thermal expansion, etc., it requires a large number of components in a somewhat complicated fabrication process and thus is of limited commercial value. More specifically, the bending of the four edges of the separator plate back upon itself is not easily accomplished particularly where two of the edges are positioned on a first side of the separator plate and the other two edges are positioned on a second, opposite side of the separator plate.
The second application referenced above is directed to a fuel cell separator having a substantially flat, planar gas-impermeable plate, and a sealing flange unitary with the plate and comprising a peripheral margin of the plate folded back upon itself and including a first portion spaced from the plate and a second portion resiliently compressed in a direction generally normal to the plane of the plate so as to provide a "bellows-effect" for accommodating variation in the spacing between the first portion of the sealing flange and the plate. This approach, while providing an effective mechanical sealing arrangement for the fuel cell, involves the use of the same material in both the electrically active, inner portion of the separator plate as well as in the peripheral, sealing portion of the separator plate. Therefore, the designer must choose between a good conductor and a highly corrosion-resistant material for use in the fuel cell separator. This choice, due to limitations in the materials currently available, results in limited cell efficiency and only moderately effective corrosion resistance.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a gaseous fuel cell separator comprised of a generally flat, center portion having high electrical conductivity and a plurality of resilient, sealing flanges attached to the edges thereof which are highly corrosion existent and provide an effective fuel cell field. The present invention provides a low cost, easily fabricated and assembled fuel cell separator which is highly conductive in the electrically active portion of the fuel cell, while forming a gas-tight wet seal around the periphery of the fuel cell.